Anders Breivik has referred to leftist plots against Europe and made confused allusions to the Knights Templar. Photograph: KeystoneUSA-ZUMA / Rex Features

In 1990s Ireland, a poster campaign was launched to encourage more enlightened attitudes to homosexuality. Alongside various pictures of happy, smiling, safe-looking women ran the caption: "Lesbians are Everywhere". It's clear what they were getting at, but you couldn't escape the notion that it sounded vaguely like a threat. Or possibly a warning: "Watch out! They're behind you, plotting in their dark way."

People could very easily have become obsessed with spotting lesbians, indeed harboured the suspicion that every woman they met was gay. After all, if lesbians are everywhere, then it seems pretty likely this woman before me is a lesbian, doesn't it? Roughly the same logic applies to conspiracism. Once you have bought the idea that conspiracies run the world, it seems reasonable to assume that everything is a conspiracy.

A glance at some of the big conspiracist websites shows there is a whole range of issues which are, absolutely, definitely conspiracies. Al-Qaida attacks on the United States, indeed al-Qaida itself, are definitely not what they seem. The banking crisis has been engineered to keep us subjugated. Water is fluoridated (or worse) to keep us docile and impotent. Harmful vaccines are distributed with malice, as part of "The Hidden Agenda for a Global Scientific Dictatorship".

Like so much information, this stuff has found new currency on the web. And it's undeniable that the web has made it easier for like-minded people to find each other. But it's certainly not a new phenomenon. Writing on conspiracist thinking in Tribune in 1944, George Orwell (a man often quoted by conspiracy theorists) noted: "In one form or another this kind of thing seems to attack nearly everybody, apparently answering to some obscure psychological need of our time."

That "obscure psychological need" seems plain enough to me. The beauty of the true conspiracy theory is that there is no room for error or stupidity. Everything is planned. That is a comforting thought. This deep faith in the existence of a masterplan is, for all the radical posturing of the modern conspiracist, a deeply conservative viewpoint. The idea of the Illuminati was thought up in the 18th century to explain why the Enlightenment rejected the pillars of church and monarchy. One hundred years later, the Protocols of the Elders of Zion were adapted by the tsarist regime in Russia, pinning blame for social unrest and anarchist violence on the easiest target – the country's already deeply oppressed Jews – who despite their desperate poverty and isolation in Russian society, apparently ran the world.

We may, in the Occupy movement, be seeing the latest strain of this tied-up thinking. The protesters have found a "They" (the nebulous 1%), who are keen to keep the masses (the 99%) subjugated. The recently published Occupy manifesto suggested not a serious critique of capitalism, but a skewed, and possibly dangerous view of how the world works. While the group seems broadly benign, it was disheartening to see their London ranks swelled by believers in "freeman of the land" pseudo law. More worryingly, the St Paul's camp was regularly frequented by advocates for Iran's Press TV, the propaganda channel of a regime that holds conspiracism at its core.

Meanwhile, on the David Icke website, 9/11 truther Luke Rodowski tells Icke he has been part of Occupy Wall Street from day one.

And here lies a real problem. Conspiracism can appeal to the disenfranchised and idealistic as an explanation of why the world isn't how you'd like it to be. But it can become dangerous when one starts to believe that the big "They" are intent on keeping you from reaching your full potential, possibly out of malice, possibly out of fear of your unleashed abilities. Start that line, and you're really not far from believing in the Übermensch, a view that Anders Breivik undoubtedly subscribed to in his hotchpotch manifesto, filled with references to leftist plots against Europe and confused allusions to the Knights Templar, possibly gleaned from Scandinavian television's regular showings of the 1982 TV film of Ivanhoe.

None of this is to suggest conspiracies don't happen, that men do not gather in secret and plot to orchestrate events, that governments will not do unspeakable things to citizens without them knowing.

There is little doubt now, for example, that the UK secret services conspired to overthrow the democratic Mossadegh government in Iran, mendacious meddling from which the Iranian people are still feeling the repercussions to this day, in the form of the Islamic Republic. Or indeed that generals have subjected soldiers to drug tests and dangerously high levels of radiation, without alerting the men to the full dangers.

It is absolutely correct to be sceptical of those in power. But conspiracism does not offer scepticism, merely an all-encompassing mush that preys on the idealistic and the gullible and turns them to paranoid cranks.

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